1. Field of the Invention
The main object of the present invention is a device and method for the transmission or storage of data, optimizing the use of the pass-band.
2. Description of the Prior Art
Efforts have been under way for many years to enable transmission of increasingly large quantities of data.
For example, for data transmission by ratio, the use of amplitude modulation and frequency modulation is leading to the use of increasingly higher carrier frequencies. Now the transmission of data in phase modulation would enable a substantial increase in throughput. However, this type of transmission comes up against two problems which, in prior art devices, limit performance to below that of devices using amplitude modulation or frequency modulation.
Firstly, to be able, at reception, to decode data encoded in phase modulation, it is essential to have a stable phase reference. Thus, it is indispensable to obtain perfect synchronization of clocks at transmission and at reception. The throughput rates of the data that can be transmitted depends on the stability of the clocks as well as on the precision of the phase comparators.
Secondly, the performance of devices for data transmission in phase modulation are limited by multiple echos. For, when using coherent waves, there is a risk that the multiple echos will interfere with the direct signal and disturb reception.
The device of the present invention enables the increase of the signal/noise ratio through the use of codes, for example, pseudo-random codes in phase modulation, this increase in the signal/noise ratio being got by pulse compression, and/or also enables the obtaining of the synchronization of the transmission and reception of clocks. Advantageously, at reception, the periodic synchronization of the clock is got by convolution and/or correlation of the received signal with a pseudo-random code. The synchronization is improved by using codes of great length.
In an alternative embodiment of the device according to the invention, regular codes such as, for example, 0, .pi., 0, .pi., 0, .pi., 0, .pi., . . . are used for the convolution.
The use of a pseudo-random code when synchronizing the reception clock ensures the confidentiality of transmissions. It is thus possible to make transmissions which can be received only by a receiver fitted with the appropriate pseudo-random code generator. Thus, it is possible, in particular, to make subscriber television broadcasts.
The multiple echos are advantageously removed by computation. Furthermore, the performance of the device according to the present invention is further increased by using a modulation by measurements of the time elapsed before a transition, for example, within periods of fixed duration.
The present invention is not limited to data transmission. It also pertains to the storage of data on a magnetic medium for example. The use of phase modulation or modulation where measurements are made of the time elapsed before a transition within periods of fixed duration makes it possible to increase storage density and obtain one and the same type of modulation throughout the transmission-reception-recording chain.